The present invention relates generally to a photovoltaic concentrator module for converting incident solar radiation to electricity and, more particularly, to a planar photovoltaic concentrator module having improved safety and reliability, improved producibility and manufacturability, and improved performance characteristics.
A photovoltaic solar concentrator module typically utilizes a lens to concentrate solar radiation onto a solar cell assembly which, in turn, converts the solar radiation into electricity. The solar cell assembly is precisely located at the bottom of a metal or plastic housing. The assembly and the housing are collectively referred to as the module. In practice, multiple solar cell assemblies are located in the housing and are electrically configured to form a module operable to generate an electrical signal having a magnitude sufficient to, for example, charge a battery or operate electrically powered equipment.
Although photovoltaic concentrator modules presently available have been used with some success, their use has not been trouble free. Typically, because of the low voltages and the high currents generated by solar cells under intense solar concentration, the solar cells of a photovoltaic concentrating solar module are electrically connected in series. Therefore, the cells must be electrically insulated from each other. Also, to increase their efficiency, the cells must be effectively cooled. Ideally, the solar cell circuit would be encapsulated in a module housing made of an inexpensive material that has low electrical conductivity, high thermal conductivity, and simple processing. Diamond and certain highly purified semiconducting materials have low electrical conductivity and high thermal conductivity. Certain ceramic materials have low electrical conductivity and moderately high thermal conductivity. However, all these materials are expensive and not easily processed.
Another problem experienced with presently available photovoltaic concentrator modules results from the use of metal contacts, metal heat spreaders, and metal heat sinks to conduct electricity and to dissipate heat from the solar cells which form "portions"--regions--of the solar cell assemblies. Because of the high electrical currents generated by typical solar cells under appreciable solar concentration, the areas of the electrical contacts must necessarily be large to reduce electrical resistance losses. Heat dissipation is desirable because it reduces the temperature of the solar cells thereby increasing efficiency. Heat spreaders conduct heat laterally away from the solar cell to reduce the intensity of this heat before it passes through materials with low thermal conductivity. Then, heat sinks convect and radiate this heat to the atmosphere. Because of the low effectiveness of passive air cooling, the area of each solar cell assembly heat sink is comparable to the illuminated area of its associated lens. In all known photovoltaic concentrator modules, past and present, some or all of these metal components are exposed either inside or outside the module housing, which poses serious safety and reliability problems, especially in the presence of moisture. Complete insulation of solar cell assemblies and heat sinks has not yet been accomplished in a satisfactory manner because of the sheer size of these components and because it is presently believed that heat sinks must be exposed to atmosphere to provide adequate cooling. It is known to employ thin plastic or ceramic films to insulate electrical contacts from heat spreaders and to insulate heat spreaders from heat sinks if the solar cell assembly is to be positioned in a metal housing; however, the performance and reliability of plastic and ceramic films has proven to be marginal.
Production of concentrator modules has proven to be difficult, costly, and time-consuming because of the use of bulky and expensive metal or plastic housings and the difficulties of handling fragile and bulky cell assemblies and of accurately positioning the cell assemblies within these housings. Bulkiness of the assemblies can be reduced by mounting the solar cell array on a plane as in U.S. Pat. No. 4,834,805, entitled "Photovoltaic power modules and methods for making same," to Erbert, issued on May 30, 1989.
In addition to the problems caused by moisture, heat, and production difficulties in presently used photovoltaic concentrator modules, concentrator modules are known to experience additional problems because of solar cell assembly/lens misalignment when metal housings are used to encase the solar cell assemblies. As previously mentioned, solar cell assemblies are typically configured in an array and a corresponding array of plastic lenses concentrates solar radiation onto the solar cell assemblies. The solar cell assemblies are mounted on the back wall of the module housing and the plastic lenses form the front wall of the housing. If a metal housing is used to encase the solar cell assemblies, a problem arises because of the differences in the thermal and moisture-induced expansion between the plastic lens array and the metal housing. These differences in thermal and moisture-induced expansion cause misalignment between the solar cell assemblies and the lenses, which reduces the amount of solar radiation directed toward the solar cells themselves. This obviously results in a reduction in the amount of electricity generated by the solar cell assemblies. In extreme cases, this misalignment may cause the lenses to physically separate from the housing, allowing moisture to freely enter the housing. This will damage the solar cells and electrical connections located within the housing.
If a plastic housing is used to encase the solar cell assemblies, problems arise because the seals around openings in the housing allowing exposure of metal heat sinks are unreliable. Moreover, housings are expensive to develop and produce because of their requirement for high dimensional accuracy, and because the cell assemblies must be laboriously mounted and fastened within these housings.
Consequently, a need exists for an improved photovoltaic concentrator module which overcomes the shortcomings of the prior art. In particular, there is a need for a photovoltaic concentrator module which is resistant to moisture and dirt and configured to allow the solar cell assemblies and heat sinks to be fully insulated without adversely affecting the module's heat dissipation. Additionally, a need exists for an improved photovoltaic concentrator module that is easier to produce than the prior art and whose performance is less sensitive to differences in temperature than the prior art.